Methods for forming an array of MEMS optical elements

ABSTRACT

An embodiment of the invention comprises an optical element capable of motion in at least one degree of freedom wherein the motion in at least one degree of freedom is enabled by serpentine hinges configured to enable the optical element to move in at least one degree of freedom. The embodiment further includes driving elements configured to deflect the optical element in said at least one degree of freedom to controllably induce deflection in the optical element and a damping element to reduce magnitude of resonances. Another embodiment includes a MEMS optical apparatus comprising an optical element capable of motion in two degrees of freedom. The two degrees of freedom are enabled by two pairs of serpentine hinges. A first pair of serpentine hinges is configured to enable the optical element to move in one degree of freedom and a second pair of serpentine hinges is configured to enable the optical element to move in a second degree of freedom. The apparatus further includes driving elements configured to deflect the optical element in said two degrees of freedom and a damping element to reduce magnitude of resonances. The invention includes method embodiments for forming arrays of MEMS optical elements including reflector arrays.

CROSS-REFERENCE TO RELATED APPLICATIONS

This U.S. patent is a continuation of application Ser. No. 10/035,829entitled “Micro-Opto-Electro-Mechanical Switching System,” by VladNovotny and Parvinder Dhillon, filed Oct. 18, 2001 now U.S. Pat. No.6,963,679, application Ser. No. 10/035,829 is:

-   -   1. a continuation of application Ser. No. 09/981,628, filed Oct.        15, 2001 now abandoned, also by Vlad Novotny and Parvinder        Dhillon;    -   2. a continuation-in-part of application Ser. No. 09/865,981,        now U.S. Pat. No. 6,483,962 B2, i ssued on Nov. 19,2002,        entitled “Optical Cross Connect Switching Array System with        Optical Feedback” filed on May 24, 2001 and invented by Vlad J.        Novotny, which claims priority under 35 U.S.C. §119(e) from U.S.        patent application Ser. No. 60/206,744, entitled, “Optical Cross        Connect Switching Array Systems With Optical Feedback Control”        filed May 24, 2000; and from U.S. patent application Ser. No.        60/241,269, entitled, “Micro-Electro-Mechanical Systems for        Optical Switches and Wavelength Routers,” by Vlad J. Novotny and        Parvinder Dhillon, filed Oct. 17, 2000.    -   3. a continuation-in-part of application Ser. No. 09/880,456,        now U.S. Pat. No. 6,625,341 B1, issued Sep. 23, 2003, entitled:        “Optical Cross Connect Switching Array System with Electrical        and Optical Position Sensitive Detection”, invented by Vlad J.        Novotny, filed Jun. 12, 2001. U.S. Pat. No. 6,625,341 B1 is a        continuation-in-part of aforementioned application Ser. No.        09/865,981, filed May 24, 2001, U.S. Pat. No. 6,483,962 B2; and        claims priority under 35 U.S.C. §119(e) from U.S. patent        application Ser. No. 60/211,239, entitled “Optical Cross Connect        Switching Array Systems With Multiple Optical And Electrical        Position Signal Detectors,” by Vlad J. Novotny, filed Jun. 12,        2000.        Each of the above-referenced patents and applications is hereby        incorporated by reference.

FIELD OF THE INVENTION

The invention generally described herein relates to the design andfabrication of micro-optic devices. In particular, the present inventionpertains to micro-electro-mechanical systems (MEMS) optical assembliesused in fiber optic switching arrays, wavelength routers, laserscanners, bar code scanners, variable optical attenuators (VOA),wavelength tunable lasers, and other related devices. More particularly,the present invention pertains to the design, structure and fabricationof MEMS reflectors and hinges used in fiber optic switching devices.

BACKGROUND

As is well known, fiber optic technology is a rapidly growing field withvastly expanding commercial applicability. As with all technologies,fiber optic technology is faced with certain practical difficulties. Inparticular, the design and fabrication of arrays of optical elementsthat enable the efficient switching and coupling between input opticalelements and output optical elements in an optical network is asignificant consideration of designers, manufacturers, and users ofoptical systems. Optical systems commonly use laser generated lightbeams, to carry information through optical fibers and are directedthrough complex optical paths with the assistance of optical switchingelements, routers and other like components. Other applications includewavelength routers that demultiplex incoming signals into individualwavelength and then switch in the nonblocking fashion single wavelengthsbetween outputs, laser beam deflectors in laser printers, bar codereading devices and others.

FIG. 1 schematically illustrates a portion of a fiber optic network 100.In the depicted embodiment, network 100 routes optical signals throughfiber optic lines L from node to node to form an interlaced ring-meshnetwork structure. Many other configurations of network structures arepossible. In the depicted embodiment, the fiber optic lines L areinterconnected at optical nodes 101, 102, 103, 104, 105, and 106. Theoptical signals are directed to their desired destination by opticalswitching. Typically, this switching is accomplished at the opticalnodes 101, 102, 103, 104, 105, and 106 (also referred to herein asswitching nodes). Each switching node 101, 102, 103, 104, 105, and 106accommodates a plurality of fiber optic lines L which comprise inputfibers and output fibers. It is the selecting of and switching betweenthese input fibers and output fibers that define the optical paths whichroute optical signals to their desired target destinations.

FIG. 2( a) is a simplified schematic illustration showing an overview ofbi directional optical cross-connect switching array system 200. Thesystem 200 includes fiber arrays 202 and 204 for passing light beamsinto and out of the switching array system 200. Each fiber array 202,204 comprises a plurality of fiber optic transmission lines (a portionof which are shown here by fibers 210, 211, 220, and 221). Forconvenience, fiber array 202 shall be referred to as an incoming fiberarray 202 and the fiber array 204 shall be referred to as an outgoingfiber array 204. However, it should be remembered that due to the bidirectional nature of the switching array system 200, the terms incomingand outgoing are relative.

Light beams carry information throughout the optical network. The lightbeams are directed to their final destination by passing throughswitching array systems 200 which direct the light beams to the desireddestination. Electronic control circuitry 230 is used to dynamicallycontrol the switch 200 configuration. The control circuitry 230 caninclude, among other elements, position sensitive detectors,demultiplexing circuitry, photodetectors, position sensing detectors,amplifiers, decoding circuitry, servo electronics, digital signalprocessors, communication hardware, and an application programminginterface. The control circuitry directs entering light beams to thedesired exit fibers.

The following simplified illustration describes how a light beam can beswitched from one of the incoming fibers in array 202 to a selected oneof the fibers in array 204. Such description is also applicable toswitching a light beam between any selected fiber in array 204 to aselected fiber in array 202.

In the depicted illustration, the light beam 231 exits the fiber 210(and in preferred embodiments, passes through a lens array (not shown)so that the beam propagates without significant divergence) onto thereflector array 218. Servo electronics of the control circuitry 230initiate deflection in a reflector 218′ of the reflector array 218 todirect the light beam 231 along an optical path 232 to a desired fiber220 (in fiber array 204) using a signal from position detection array234. By changing the deflection of the reflectors (e.g., 218′) of thereflector array the light beams can be switched to enter any selectedoutgoing fiber 204. Also, the deflection of each of the reflectors 218′can be altered in very small ways to fine tune light beam opticalcharacteristics. The reflector 218′ deflection can be adjusted inresponse to instructions contained within the data streams of the lightbeam 231. Alternatively, reflector 218′ deflection can be adjusted inresponse to instructions provided externally via an applicationprogramming interface of, for example, the control circuitry. Othermethods of adjusting reflector 218′ deflection known to those havingordinary skill in the art can also be used.

A light beam can be switched from one outgoing fiber to another outgoingfiber, by changing reflector deflection angle. For example, if lightbeam 231, 232 is to be switched from fiber 220 into another outgoingfiber 221, the controller circuitry 230 sends appropriate instructionsto the servo electronics which reposition the reflector 218′ so thatbeam 231 is redirected along optical path 233 to fiber 221. Typically,the beams (e.g., 232, 233) pass through a lens array (not shown) whichfocuses and couples the light beam (here 233) into the outgoing fiber(here 221). It should be noted that although fibers have heretofore beenreferred to as belonging to the incoming fiber arrays 202 or theoutgoing fiber arrays 204, such fiber arrays are bi-directional. In suchbi-directional embodiments, light beams also travel from the outgoingfibers in the outgoing fiber array 204 to incoming fibers in theincoming fiber array 202. This is done in the same way as light beamstraveling from incoming fibers in the incoming fiber array 202 tooutgoing fibers in the outgoing fiber array 204. Also shown in FIG. 2(a) are the position-sensitive-detectors 234, which feed theposition-error-signals to the controller circuitry 230.

The switching array system 200 is shown as one-dimensional in theembodiment of FIG. 2( a) for clarity. In preferred embodiments theaforementioned arrays are two dimensional. For example, in an embodimentwith a two-dimensional reflector array 218, there are rows and columns,or some other two-dimensional arrangement of reflectors. The otherarrays and alignment structures are similarly two-dimensional in someembodiments. In addition, the overall system is shown as two-dimensionalin FIG. 2( a). In preferred embodiments, the system isthree-dimensional, as the additional dimension in and out of the planeof the paper can be advantageously used to position the variouscomponents and minimize the dimensions of the hardware.

It should be noted that although FIG. 2( a) depicts the switching device200 as having a single reflector array 218, many embodiments include twoor more reflector arrays instead of just one with or without additionalplane reflectors. One such embodiment is schematically illustrated inFIG. 2( b). FIG. 2( b) is a simplified schematic illustration showing anoverview of two-reflector array bi-directional optical cross-connectswitching array system 201. The system 201 includes fiber arrays 202 and204 for passing light beams into and out of the switching array system201. The fiber arrays 202, 204 include a plurality of fiber optictransmission lines (a portion of which are shown here by fibers 210,211, 220, and 221). Here, the incoming light beam 234 is directed towarda first reflector array 217 which reflects the beam 234, 235 onto asecond reflector array 219 and then into the desired outgoing fiber(here, 221). Switching may be accomplished by altering the deflection ofthe reflectors of the first reflector array 217 or by altering thedeflection of the reflectors of the second reflector array 219 or byaltering the deflection of the reflectors of the first reflector array217 and the reflectors of the second reflector array 219 at the sametime. In this example, the path of light beam 234 is altered by thedeflection of first reflector 217′ which directs the light beam 234 ontothe altered beam path 236 onto second reflector 219′ and into outgoingfiber 220. Additionally the control circuitry (not shown) controls thereflectors of both the first reflector array 217 and the secondreflector array 219. Although structurally somewhat different from thepreviously discussed embodiment 200, the principles of operation of suchmultiple reflector array switches 201 are similar. Similar switchingfunctions can be performed using alternative switching configurations.For example, one embodiment can use combined first and second sets ofmovable reflectors and one fixed reflector. An optical beam can beswitched by reflection of an input beam from a movable reflector onto afixed reflector and from this reflector back onto a movable reflectorand into output fiber. Number of reflectors in the combined array is thesame as total number of reflectors in two physically separate arrays.Many other configurations are used and known by those having ordinaryskill in the art.

MEMS switching arrays can also be used in wavelength routers. Oneembodiment of such a wavelength router is depicted in FIG. 3. Usingwavelength division multiplexing light beams of several wavelengths canbe optically transmitted using the same fiber. For example, a singlefiber can carry light beams comprising k signals at k wavelengths. Theselight beams of many wavelengths are coupled from a fiber 331 into awavelength division demultiplexer 334. The demultiplexer 334 can bebased on arrayed waveguide gratings, interference filters, or fiberBragg gratings. The illustration of FIG. 3 uses an arrayed waveguidegrating 334 as a wavelength division demultiplexer. Multi wavelengthlight beam 345 enters into the first free space region 335, is separatedinto individual wavelengths in grating 333 and exits through the secondfree space region 336 where light beams at k wavelengths are spatiallyseparated. Light at each specific wavelength is coupled into linearfiber array that directs light beams onto a lens array 342. Relativelycollimated light beams such as 343 and 344 propagate toward mirrors ofthe first array 337. The light at each specific wavelength is reflectedfrom one mirror in the first array 337 onto a specific mirror of thesecond mirror array 338 from which the light is directed onto focusinglenses 339 and into a selected output fiber 351. The mirror arrays 337and 338 can be one-dimensional arrays in order to match the spatialdistribution of the light beams or two-dimensional arrays. Mirror arrays337 and 338 are formed by bi-axial (bi-axially actuated) mirrors.

SUMMARY

In accordance with the principles of the present invention, oneembodiment of the invention comprises an optical element capable ofmotion in at least one degree of freedom wherein the motion in at leastone degree of freedom is enabled by serpentine hinges configured toenable the optical element to move in the at least one degree offreedom. The embodiment further includes driving elements configured todeflect the optical element in said at least one degree of freedom tocontrollably induce deflection in the optical element and a dampingelement to reduce magnitude of resonances

Another embodiment includes a MEMS optical apparatus comprising anoptical element capable of motion in two degrees of freedom. Thesedegrees of freedom are enabled by a first pair of serpentine hinges thatis configured to enable the optical element to move in one degree offreedom and a second pair of serpentine hinges that is configured toenable the optical element to move in a second degree of freedom. Theapparatus further includes driving elements configured to deflect theoptical element in said two degrees of freedom and a damping element toreduce magnitude of resonances.

Another embodiment includes a MEMS optical apparatus comprising incombination a support structure, a movable optical element, at least onepair of serpentine hinges, driving elements positioned such thatactivation of the driving elements can controllably induce deflection inthe movable optical element and a damping element. The combinationcomprising means for inducing a damped rotation of the movable opticalelement about an axis of rotation defined by each of the at least onepair of serpentine hinges.

A method embodiment for forming an array of MEMS optical elementscomprises: providing a silicon-on-insulator (SOI) wafer. Photoresistmasking the top and bottom surfaces with appropriate patterning. Firstetching to remove the top oxide layer in hinge regions defined by theopenings in the top photoresist layer exposing a hinge region of thedevice silicon layer. Forming a second photoresist layer patterning thehinge region of the device silicon layer so that a hinge can be formed.Second etching the patterned hinge region to remove portions of thedevice silicon layer forming recessed portions and such that unetchedsurfaces correspond to a hinge. Removing the second photoresist layer,thereby exposing the underlying top oxide layer as a hard mask layerhaving openings in the hinge region. Third etching the device siliconlayer through the openings in the hard mask wherein the recessedportions are etched until the internal oxide layer is reached whereinthe previously unetched surfaces are partially etched leaving a portionof the unetched surfaces in place as hinges. Fourth etching the bottomsurface of the SOT wafer to form a pocket region and a separation lineregion. Fifth etching the SOT wafer to remove the internal oxide layerin the pocket region. Forming a reflective layer on at least one surfaceof the movable optical element, and a sixth etching to remove materialfrom the separation line region to complete the separation line therebyenabling the substrate to be separated into arrays of a desired size.

These and other aspects and advantages of the invention will becomeapparent from the following detailed description and accompanyingdrawings which illustrate, by way of example, the principles of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the invention, reference is made to theaccompanying drawings in the following Detailed Description. In thedrawings:

FIG. 1 is a figurative illustration of an optical network.

FIGS. 2( a) and 2(b) are simplified schematic illustrations of a singlereflector array and two reflector array optical switch embodiments.

FIG. 3 is a simplified schematic illustration of an embodiment of awavelength router.

FIG. 4( a) is a top down view of an embodiment of a reflector array.

FIGS. 4( b) and 4(c) are top down views of an embodiment of a reflectorassembly.

FIGS. 4( d) and 4(e) are cross section views of a portion of theembodiment shown in FIGS. 4( b) and 4(c).

FIGS. 5( a)–5(e) are top down views of serpentine hinge embodiments inaccordance with the principles of the present invention.

FIG. 5( f) is perspective view of a serpentine hinge embodiment inaccordance with the principles of the present invention.

FIGS. 5( g), 5(h) and 5(i) are plan and cross-sectional views of thehinge embodiments having damping material applied in accordance with theprinciples of the present invention.

FIGS. 6( a) and 6(b) are plan views of a reflector assembly embodimentin accordance with the principles of the present invention particularlydepicting frame, mirror, and serpentine hinge elements and theassociated underlying driving elements.

FIGS. 7( a) and 7(b) are plan views of another reflector assemblyembodiment in accordance with the principles of the present inventionparticularly depicting frame, mirror, and radial serpentine hingeelements and the associated underlying driving elements.

FIG. 7( c) is a plan view of a radial serpentine hinge in accordancewith the principles of the present invention.

FIGS. 8( a) and 8(b) are plan views of another reflector assemblyembodiment in accordance with the principles of the present inventionparticularly depicting frame, mirror, and circumferentially curvedserpentine hinge elements and the associated underlying drivingelements.

FIG. 9( a) is a plan view of a variable spring constant serpentine hingeembodiment in accordance with the principles of the present inventionFIGS. 9( b) and 9(c) are plan views of another reflector assemblyembodiment in accordance with the principles of the present inventionparticularly depicting frame, mirror, and circumferentially curvedvariable spring constant serpentine hinge elements and the associatedunderlying driving elements.

FIGS. 10( a) and 10(b) are plan views of another reflector assemblyembodiment in accordance with the principles of the present inventionparticularly depicting multiple frames, mirror, straight hinge elements,serpentine hinge elements, and the associated underlying drivingelements.

FIGS. 11( a)–11(m) depict a series of cross-section views of a substrateupon which a reflector embodiment is being formed in accordance with theprinciples of the present invention, each Figure illustrating varioussteps of a fabrication process.

Reference numerals refer to the same or equivalent parts of the presentinvention throughout the several figures of the drawings.

DETAILED DESCRIPTION

The present invention has been particularly shown and described withrespect to certain preferred embodiments and specific features thereof.The embodiments set forth herein below are to be taken as illustrativerather than limiting. It should be readily apparent to those of ordinaryskill in the art that various changes and modifications in form anddetail may be made without departing from the spirit and scope of theinvention.

FIG. 4( a) figuratively depicts a plan view of an embodiment of an M×Narray 400 of movable optical elements 401 (where M and N representinteger values from 1 to m and from 1 to n, respectively). Where themovable optical elements 401 are reflectors, such an array 400 can beincorporated into an optical switching device in accordance with theprinciples of the present invention. The array contains a plurality ofmovable optical elements 401 formed on the substrate or supportstructure of the array 400. These optical elements can comprise a widerange of optical components including, but not limited to reflectors(mirrors), blocking optics (which block the transmission of light),filters, gratings and lenses. Such movable optical elements serve anumber of purposes and can be incorporated into numerous optical devicesincluding optical switches. Such movable optical elements 401 can bemovable about one axis or about two axes (so-called bi-axial opticalelements). Throughout this patent these movable optical elements 401will be discussed in the context of reflectors. It should be appreciatedby those having ordinary skill in the art that the movable opticalelements 401 described herein as reflectors can be interchanged withother optical elements, including but not limited to any of theaforementioned optical elements. Thus, the optical element array of FIG.4( a) will be described as reflector array 400. Each reflector array 400includes M×N reflector assemblies 401 formed on the substrate structureof the reflector array 400. The inventors contemplate many uses for suchreflector arrays including, but not limited to single reflector arrayswitching devices and two reflector array switching devices, as well aswavelength routers incorporating single or double reflector arrays.

FIG. 4( b) schematically illustrates aspects of a bi-axial reflectorassembly 401 capable of deflection in two degrees of freedom.Embodiments for rotating in one degree of freedom are also contemplatedby the inventors. The depicted reflector assembly 401 includes areflective element 407, commonly referred to as a mirror. The mirror 407is supported in a frame 408 by a pair of mirror hinges 404. The hingesdepicted here are schematic in nature. The preferred hinge embodimentsare discussed in greater detail below. The pair of mirror hinges 404supports the mirror 407 such that an axis of rotation (here, forexample, rotation about an X-axis) is defined. The frame 408 issupported in the substrate structure 405 of the reflector array 400 byanother pair of frame hinges 406. Typically, the mirror 407 ispositioned inside a recess in the substrate structure 405 of thereflector array 400 such that the mirror 407 has clearance to be tilted.Alternative embodiments for the mirror 407 position the mirror 407 sothat it is raised above the surface of the substrate structure 405 ofthe reflector array 400.

The pair of mirror hinges 406 supports the frame 408 such that anotheraxis of rotation (here, for example, rotation about a Y-axis) isdefined. Typically, the pairs of hinges 404, 406 define substantiallyperpendicular axes of rotation. Thus, three-dimensional motion can beachieved in the reflector assemblies 401. Simpler, reflector assembliescan also be constructed. Such assemblies only rotate about a singleaxis. These reflector assemblies find utility in many applicationsincluding smaller optical switches and in so-called digital (on-off)switching arrays. Such arrays only require rotation about a single axis.Generally speaking, such reflector assemblies 401 are driven byelectrostatic, electromagnetic, piezoelectric or thermal drivingelements. Electrostatic actuators are commonly fabricated underneath themirror 407 and frame elements 408. These driving elements are typicallycontrolled by the control circuitry of the switch. The control circuitrythat drives the driving elements can be formed directly underneath thedriving elements as part of the fabrication process or elsewhere on thereflector array 400. Alternatively, the control circuitry that drivesthe driving elements can be formed completely separate from the array407 and connected later.

FIG. 4( c) schematically illustrates driving elements (or driveelements) 407′, 407″, 408′, and 408″. In the depicted illustration, thedrive elements are, for example, positioned beneath the moving parts.The driving elements 407′, 407″ rotate the mirror 407 about the X-axis,and driving elements 408′, and 408″ rotate the frame 408 about theY-axis. The driving elements 407′, 407″, 408′, and 408″ are typicallyconstructed of parallel plate capacitors. Driving control electronicscan be included below driving elements 407′, 407″, 408′, and 408″ forlarger reflector arrays and connected to the driving elements using, forexample, vias. In other embodiments, the driving electronics can be onseparate wafers and leads can be routed on the surface to the drivingelements 407′, 407″, 408′, and 408″.

FIG. 4( d) is a cross section view of a portion of the embodiment shownin FIG. 4( c). The driving elements 407′, 407″, 408′, 408″ depicted inthis embodiment is preferably constructed having a slightly smaller sizethan the frame 408 and reflector 407 elements. During operation, theframes and mirror elements have been known to rotate too much causingthe outer edges of the frames and mirror elements to make mechanicalcontact with the underlying drive elements. This electrically shortcircuits the system and can permanently damage the system components.Thus, if the driving elements have a slightly smaller dimension,especially along the outer edges, excessive deflection of the frames andmirror elements will not result in shorting of the system. As depictedin FIG. 4( c), the drive elements are smaller (especially at the distaledges) than the movable elements, as indicated by the heavy boundarylines. Therefore, even if the movable elements contact the underlyingstructure there will be no electrical contact between the drive elements407′, 407″ and the depicted reflector 407. The same can be said of driveelements 408′, 408″, and frame 408. Such sizing of the drive elementscan be utilized with respect to all the embodiments set forth herein.Implementation in FIG. 4( d) has the gap 415 between mirror element 407and frame 408 and driving elements 407′, 407″, 408′ and 408″ defined bythe thickness of the wafer 412 minus thickness of mirror 407. Whensmaller gaps 416 shown in FIG. 4( e) are required, the bottom chip 414with driving elements is etched with trenches 417. Metal coatings 410and 411 are also shown in FIGS. 4( d) and 4(e). The frame 408 does notrequire metal coating as the silicon material is extrinsic, havingdopants in silicon for all these devices. When wafer 414 is electricallyconducting, dielectric film is deposited on its surface in areas wherethe physical contact occurs between 412 and 414 in order to electricallyinsulate parts 412 and 414.

Although depicted here as parallel plate electrostatic actuators,driving elements in accordance with the principles of the presentinvention may be of many different types of actuators known to thosehaving ordinary skill in the art can be used. For example, other typesof actuators such as electrostatic rotational comb actuators,electromagnetic actuators, piezoelectric actuators or thermally drivenactuators can be used. Although the depicted embodiment shows the mirror407 as circular, the mirror 407 can have any shape.

Hinge design is an important aspect of the high performance reflectorassemblies. The length, width, thickness, and cross sectional shape ofhinges determine the stiffness and consequently the driving signals(voltages in case of electrostatic actuators) required to achievedesired deflections in the reflectors and the desired frequency responseof the actuator. The torsional hinge stiffness is proportional to hingethickness, to the third power of hinge width and inversely proportionalto hinge length. The bending hinge stiffness is proportional to thethird power of hinge thickness, to hinge width and inverselyproportional to hinge length. The hinge stiffness has to be low enoughto provide sensitive deflections but also high enough to exhibit highfrequency resonances. The hinge must also be robust enough to bemanufactured with a high yield and withstand the conditions of a normaloperating environment. Additionally, if the reflector was constructed sothat both the mirror and the hinges have the same low thickness, thelack of flatness of the mirror would lead to excessive wavefrontdistortions in light reflected by the mirror. Consequently, in mostcases, the mirror thickness will be greater than hinge thickness.Therefore, fabrication processes should be capable of generating thesetwo different thicknesses. In addition, hinge width is limited byprocessing (lithography and etching) and reasonable widths do not leadto acceptably low stiffness, unless the length of hinges is much greaterthan that which straight hinges can provide.

The principles of the present invention address this problem by using aserpentine hinge structure. Serpentine hinges can include one, two,three, four or more “windings”. The inventors contemplate that nwindings can be used in the hinges, where n is equal to or greater thanone. FIGS. 5( a)–5(d) depict examples of such single (5(a)), double(5(b)), triple (5(c)), and quadruple (5(d)) serpentine hinges. FIG. 5(e) shows a single winding hinge 520. Each winding includes two arms(shown here inside the dashed line boxes 521, 522). In embodimentshaving many windings, the arms snake continuously from one arm toanother for the entire length of the hinge. The windings include a pairof shafts 523, 524 which connect the hinges 520 to the larger arrayelements. The arms 521, 522 extend in a direction transverse to that ofthe axis of rotation for the hinge. FIG. 5( f) is a perspective view ofa portion of a double serpentine hinge 500. The thickness 501 of thehinge typically ranges from about 3 (micron) to about 50 km. Thickerembodiments can be fabricated and, for some embodiments, are favored.However, the most preferred thickness is in the range of about 5–10 Thisis in comparison to a typical mirror embodiment which is in the range ofabout 10–50 thick. The width 502 of the hinge typically ranges fromabout 2 to about 10 Again, embodiments having greater widths can befabricated. However, the preferred width is in the range of about 5–10The length (defined here by the dashed line) 503 of the serpentine hinge500 can be any length. Embodiments having lengths in the 100's or even1000's of microns being preferred. Of course, the length 503 of theserpentine hinge 500 depends on the number of windings in the hinge.

Damping is an advantageous feature that can add to the utility of eachof the embodiments disclosed herein. One example of such a means is athin coating of a damping agent applied onto the hinges. Such dampingagents when dried (or cured) act as a damping factor which reducesresonances in the optical structures disclosed herein. Such dampingagents are typically polymeric materials. Suitable materials include,but are not limited to silicones and elastomer materials for example,di-methylsilicone, polyurethane, polyisobutene-co-isoprene, andpolybutadiene-co-acrylonitrile. Such damping agents are coated onto thehinges and cured. Alternatively, the damping agents are dried until thevolatile constituents outgas. Typically, such damping agents are appliedonto the hinges and part of the adjoining support structures. Suchdamping agents can be applied using, for example, an ink jet dispensingin any desired pattern and quantities over the hinge surfaces. Curingcan be with room temperature, elevated temperature or exposure toultraviolet radiation, electron beams, or a combination of thesemethods. In some embodiments, the damping agent is applied to the hingein smaller quantities, forming isolated “islands” of damping material onthe surface of the hinges. The amount of material applied to the hingescan depend on many factors, including, material type, amount ofadjustment necessary, thickness of material, method of application, andother factors.

FIG. 5( g) is a drawing showing a layer of the viscoelastic material 551applied over the hinge 520 between the frame 552 and the adjoiningsupport structure 553. This material is applied to fine tune the deviceperformance after its fabrication by providing means of adjusting thedamping while monitoring the device characteristics. The viscoelasticsheet has an adhesive coating on one side and the appropriately sizedpieces are applied over the hinge area.

FIG. 5( h) shows a variation on the use of the viscoelastic material 561so as to cover only the area of the hinge 520. Application method isbased on ink jet dispensing in any desired pattern and quantities overthe hinge surfaces. Curing can be with room temperature, elevatedtemperature or exposure to ultraviolet radiation, or a combination ofthese methods.

FIG. 5( i) shows yet another variation on the extent of coverage of theviscoelastic material 571 over the hinge 520. In this case the materialis applied to the hinge in smaller quantities, forming isolated islands571. The amount of material applied to the hinge will depend on manyfactors, including, material type, amount of adjustment necessary,thickness of material, method of application, and other factors. Someexamples of the elastomer materials are Di-methylsilicone, polyurethane,polyisobutene-co-isoprene, and polybutadiene-co-acrylonitrile.

FIG. 6( a) is a plan view illustrating one embodiment of a reflectorassembly 600 in accordance with the principles of the present invention.The mirror 607 is held in the frame 608 by a pair of serpentine mirrorhinges 609 (or mirror hinges). The serpentine mirror hinges 609 aredepicted as having two windings. Other embodiments can include 1 to nwindings. The frame 608 is suspended in the array substrate 603 by asecond set of serpentine hinges 606 (also referred to as frame hinges).As with the mirror hinges 609, the frame hinges 606 can comprise from 1to n windings

FIG. 6( b) shows a drive assembly 601 which lies just underneath themirror/frame/hinge structure depicted in FIG. 6( a). FIG. 6( b) depictsthe drive elements 607′, 607″, 608′, and 608″ that are the components ofelectrostatic actuators. The drive elements 607′, 607″ interact with themirror 607 to provide positive and negative deflection about a axis. Thedrive elements 608′, 608″ interact with the frame 608 to providepositive and negative deflection about an X-axis. The drive elements607′, 607″, 608′, and 608″ are shaped such that they do not interferewith the hinges 609 and 606. This typically means that the driveelements 607′, 607″, 608′, and 608″ are not formed under the hinges.Also, it is preferred that the drive elements 607′, 607″, 608′, and 608″be sized such that, in the event of excessive deflection of the movablemirror and frame components, no contact is made between the driveelements and the movable components. This is typically avoided byreducing the size of the drive elements such that the outer edges of themovable components will not contact the drive elements even in the eventof excessive deflection. Thus, the drive elements 607′, 607″, 608′, and608″ are slightly smaller, in the regions 654, 655, 656, 657,respectively, than the overlying mirror 607 and frame 608. Suchprecautions can be implemented into each embodiment discussed herein.

Another advantageously constructed embodiment is depicted in FIG. 7( a).This embodiment meets the challenge of packing the mirror 707, two setsof hinges 704 and 706, and frame 708 into as small an area as possible,so that optical components can be smaller and the overall dimensions ofthe cross connect switching system can be reduced. FIG. 7( b) shows alayer of the reflector assembly 701, which is formed just underneath themirror/frame/hinge structure depicted in FIG. 7( a). FIG. 7( b) depictsthe drive elements 707′, 707″, 708′, and 708″. As discussed above withrespect to the embodiment of FIG. 6, the drive elements 707′, 707″interact with the mirror 707 to provide deflection about a first axis.And the drive elements 708′, 708″ interact with the frame 708 to providedeflection about a second axis. The fabrication of such structures willbe discussed in some detail hereinbelow.

With continued reference to FIG. 7( a), by packing more opticalcomponents on a given reflector array, smaller arrays may beconstructed. Smaller reflector arrays allow a larger number of devicesto be built on a given wafer, thus reducing the cost of these reflectorarrays and the deflection angles required for switching. Furthermore,smaller structures have higher resonance frequencies, which improveswitching and addressing times for the reflector array. Also, smallerreflector arrays enable shorter optical paths within switching devices.Due to the shorter optical paths possible with such embodiments, lowerresolution position sensing systems can be used, thereby reducing cost.The depicted pairs of serpentine hinges 704, 706 each have two windings.In order to achieve more compact serpentine hinges, portions of thewindings are folded into a rectangular conformation, with the arms ofeach winding being fabricated to include proximal folds that areoriented such that they are parallel to the axis of rotation. Anembodiment of such a radial serpentine hinge 704 is depicted in FIG. 7(c). The hinge 704 permits rotation (shown by the arrow) of the mirror707 about axis of rotation. In the previous embodiment, the arms of eachwinding extend in a direction transverse to the axis of rotation. In thedepicted embodiment, the arms 710, 711, 712, 713 of each winding areformed such that a portion of the arms (also referred to as the foldedportion) extends approximately parallel to the axis of rotation. In thedepicted embodiment, the inner folded arms (e.g., 712 and 713) areshorter than the outer folded arms (e.g., 710 and 711). In otherembodiments having more windings, the arms are progressively longer andlonger, the further the folded arms are from the axis of rotation x. Oneobjective of “folding” the windings is to maintain the length of thehinge in a more compact space. Thereby, the desired degree offlexibility in the hinge is maintained in a small space. Another way ofdescribing the pairs of radial serpentine hinges 704, 706 is to say thatthe windings of the hinges have parallel arms. This means that the armsof the hinges extend in a direction substantially parallel to the axisof rotation. This is in contrast to the arms of an embodiment like thatdepicted in FIG. 5( a) where the arms can be said to be transverse tothe axis of rotation.

Another embodiment 800 is depicted in FIG. 8( a). This embodiment isalso capable of compactly arranging a mirror 807, two sets of hinges 804and 806, and frame 808 into as small area as possible. In the depictedembodiment the pairs of serpentine hinges 804, 806 are circumferentiallycurved. Such circumferentially curved serpentine hinges 804, 806 aregenerally contoured to coincide with the shape of the outside edge ofthe mirror 807. Each of the depicted circumferentially curved serpentinehinges 804, 806 has four windings comprising a circumferentially curved“quad” serpentine hinge. As with the other embodiments the hinges canhave any number (n) of windings.

FIG. 8( b) shows a layer 801 of the reflector assembly 800 which liesjust underneath the mirror/frame/hinge structure depicted in FIG. 8( a).FIG. 8( b) depicts the drive elements 807′, 807″, 808′, and 808′. Aswith the previous embodiments, the drive elements 807′, 807″ interactwith the mirror 807 to provide deflection about a first axis. Also, thedrive elements 808′, 808′ interact with the frame 808 to providedeflection about a second axis.

FIG. 9( a) illustrates another preferred hinge embodiment. The depictedhinge 900 is a variable spring constant serpentine hinge. Such avariable spring constant serpentine hinge causes vibrational damping inthe hinge. In some embodiments the implementation of such damping meansis highly desirable. The depicted hinge 900 includes four windings. Thehinge 900 begins with the longest arms on the winding at one end of thehinge 900 and the shortest arms at the other end of the hinge 900. Thearms of each successive winding are progressively shorter than that ofthe previous winding. Thus, winding 922 is shorter than winding 921. Inlike manner, winding 923 is shorter than winding 922 and winding 924 isshorter than winding 923. Such variable spring constant serpentinehinges 900 improve the resonant and vibrational behaviour of the opticalelements suspended by the hinges. As with other hinges discussed herein,the number of windings is variable and determined by the designer priorto fabrication. The variable spring constant serpentine hinges 900 canbe applied to any of the embodiments discussed herein. Such hinges haveparticular utility when applied to embodiments like that depicted inFIG. 9( b).

FIG. 9( b) depicts a reflector assembly embodiment 901 having pairs ofvariable spring constant serpentine hinges 904, 906. As with theembodiment of FIG. 8( a) the hinges are circumferentially curved. Inaddition to being generally contoured to coincide with the shape of theoutside edge of the mirror 907, the circumferentially curved serpentinehinges 904, 906 are constructed such that they demonstrate a variablespring constant in the hinges. Each of the hinges 904, 906 of depictedembodiment includes two windings. As with all of the other embodimentsdiscussed herein, the hinges can comprise any number of windings. In thedepicted embodiment, the arms of the windings nearest to the mirror 907are longer than the arms of the windings further from the mirror 907. Inembodiments having a greater number of windings in the hinges, thewindings are formed of progressively shorter arm lengths until thedesired resonance and vibration behavior is obtained for the hinge. FIG.9 shows driving electrodes corresponding to reflector in FIG. 9( b).Electrodes 907′ and 907″ are used to deflect the mirror 907 whileelectrodes 908′ and 908″ are used to deflect the frame 908.

Another reflector assembly 1000 embodiment is depicted in FIG. 10( a).FIG. 10( a) depicts an embodiment utilizing combinations of serpentinehinges 1071, 1072 and short straight hinges 1081, 1082. FIG. 10( a) is aplan view illustrating one embodiment of a reflector assembly 1000 inaccordance with the principles of the present invention. As with theprevious embodiments, the reflector assembly 1000 typically isincorporated into an array of reflectors. Each reflector assembly 1000is fabricated on a reflector array substrate 1065.

The embodiment 1000 includes a first frame 1010 which connected to thesubstrate 1065 by a pair of first serpentine frame hinges 1071 whichallows the first frame 1010 to rotate about a first axis defined by thefirst serpentine frame hinges 1071. The first frame 1010 is constructedhaving an inside periphery 1100 and an outside periphery 1100′. Thefirst serpentine frame hinges 1071 connect the outside periphery 1100′of the first frame 1010 to the substrate 1065. Positioned inside thefirst frame 1010 is a second frame 1008. The second frame 1008 includesan inside periphery 1080 and an outside periphery 1080′. The secondframe 1008 is suspended and supported by a pair of first straight hinges1081 that allow the second frame 1008 to rotate about an axissubstantially parallel to the first axis defined by the pair of firstserpentine frame hinges 1071. Positioned inside the second frame 1008 isa third frame 1009. The third frame 1009 also includes an insideperiphery 1090 and an outside periphery 1090′. The third frame 1009 issuspended and supported by a pair of second serpentine frame hinges 1072which connects the outside periphery 1090′ of the third frame 1009 tothe inside periphery 1080 of the second frame 1008. The pair of secondserpentine frame hinges 1072 allows the third frame 1009 to rotate abouta second axis defined by the pair of second serpentine frame hinges1072. The second axis is typically transverse to the first axis. In apreferred embodiment the second axis is at a substantially right angleto the first axis. Positioned inside the third frame 1009 is a mirror1007. The mirror 1007 includes an outside periphery 1070. The mirror1007 is suspended and supported by a pair of second straight hinges 1082that allows the mirror 1007 to rotate about an axis substantiallyparallel to the second axis defined by the pair of second serpentineframe hinges 1072.

FIG. 10( b) shows a layer of the reflector assembly embodiment 1001which lies just underneath the mirror/frame/hinge structure depicted inFIG. 10( a). FIG. 10( b) depicts the multiple drive elements of theembodiment 1001.

Drive elements 1007′ and 1007″ interact with the mirror 1007 to providepositive and negative deflection about the second axis. Drive elements1009′ and 1009″ interact with the third frame 1009 to provide addedpositive and negative deflection about the second axis.

Drive elements 1008′ and 1008″ interact with the second frame 1008 toprovide positive and negative deflection about the first axis. Driveelements 1010′ and 1010″ interact with the first frame 1010 to provideadded positive and negative deflection about the first axis.

As previously discussed, the drive elements are shaped and sized suchthat they do not interfere with the operation and range of motion of thehinges 1071, 1072, 1081, 1082. This typically means that the driveelements 1007′, 1007″, 1009′, 1009″, 1008′, 1008″, 1010′, and 1010″ havesmall cut out regions under the hinges such that they do not impedehinge operation. Also, as previously discussed, the drive elements1007′, 1007″, 1009′, 1009″, 1008′, 1008″, 1010′, and 1010″ can be sizedsuch that in the event of excessive deflection of the movable components(e.g., the mirror and frames), no contact is made between the driveelements and the movable components of the reflector assembly 1000.

The inventors contemplate that the serpentine hinges (e.g., hinges 1071,1072) shown in the embodiments depicted in FIG. 10( a) and FIG. 10( b)can easily be replaced by other serpentine hinge designs. For example,suitable replacements can be the radial serpentine hinge 704 depicted inFIG. 7( a) or the variable spring constant serpentine hinge 900 of FIG.9( a). Such embodiments are to be taken as illustrative examples ratherthan limitations. Also, the hinges of the embodiments depicted FIGS. 10(a) and 10(b) can be treated with damping agents to improve vibrationaland resonance behavior.

The structures disclosed herein can be can be fabricated out of siliconbased materials using MEMS surface or bulk micromachining technologies.Examples of such fabrication techniques are discussed in many standardreferences. Examples include “Silicon Micromachining” (1998) byElwenspoek, M. and Jansen, H. V.; “An Introduction toMicroelectromechanical Systems Engineering” (2000) Nadim, M.; “Handbookof Microlithography, Micromachining, and Microfabrication” (1997)Rai-Choudhury, P. Also, a suitable method of manufacture is discussed inthe paper “A Flat High-Frequency Scanning Micromirror” (2000)Solid-State Sensor & Actuator Workshop, Hilton Head, S.C., Jun. 4–8,2000 by Conant, R. A., Nec, J. T., Lau, K. Y., and Muller, R. S.

Extension of these general fabrication principles from uni-axialactuators to bi axial actuators, and from structures where both thereflector and the hinge have the same thickness to devices where thereflector and hinge thicknesses are different presents a challengingfabrication problem. This is important because, it is desirable to haverelatively thin hinges, otherwise the hinge stiffness can be too highrequiring large torque to produce the desired deflection angles, whichin turn leads to high driving signals. However, if the same lowthickness is used for the reflectors, metal coating stress and/or oxidestress can result in excessive mirror distortion. Therefore, afabrication process that permits the decoupling of reflector and hingethicknesses is advantageous. In addition, release and separation ofthese fragile bi-axial actuators requires special release and separationtechniques.

FIGS. 11( a)–11(m) illustrate a series of cross-section views of asubstrate at selected points in a fabrication process. The process isdepicted with respect to, for example, the device shown in FIG. 6( a)with cross section along line 610. The depicted fabrication processembodiment can be used to construct bi-axial actuators having hingethickness less than reflector thickness. Alternatively, the hinges canbe fabricated having hinge thickness approximately the same as reflectorthickness. Also, the depicted embodiment is shown having serpentinehinges. The same processes can be used to fabricate ordinary torsionalor bending hinges.

The depicted method embodiment illustrates a fabrication method using asingle layer silicon-on-insulator (SOI) wafer. Referring to FIG. 11( a),a suitable single layer SOI wafer 1101 can be fabricated by oxidationand bonding of silicon wafers 1104. These wafers can be treated usingknown processes to produce SOI wafer 1101 having a device silicon layer1102, internal oxide layer 1103, and silicon wafer layer 1104. A typicalthickness of wafer 1104 being on the order of about 300 to 500 um,although wafers having other thicknesses can be used. The internal oxidelayer 1103 is fabricated on the wafer layer 1104. The oxide layer 1103can be fabricated by a variety of processes known to those havingordinary skill in the art to a thickness of less than 2 um. The devicesilicon layer 1102 can then be fabricated on the oxide layer 1103 bygrinding, lapping and polishing to a thickness in the range of about 1um to 100 um, with 20 to 50 um being preferred. Other fabricationmethods of SOI wafers can also be employed with particular emphasis onfabrication processes that permit the layer 1103 to be of low stress.Layer 1103 can be fabricated using materials other than silicon dioxide,such materials include, but are not limited to silicon nitrides, siliconoxynitrides, aluminum oxides, and other materials that form good bondingwith silicon and are good etch stops in reactive ion etching of silicon.

In FIG. 11( b) both sides of the SOI wafer are treated to form a topoxide film layer 1111 and a bottom oxide film layer 1112. The top oxidefilm layer 1111 and a bottom oxide film layer 1112 are typically eachformed to a thickness of less than or equal to 3 um.

In FIG. 11( c) a bottom photoresist layer 1113 is formed on the bottomoxide film layer 1112. The photoresist layer 1113 has openings defininga bottom pocket 1116 and a separation line 1117. In FIG. 11( d) a topphotoresist layer 1114 is formed on the top oxide film layer 1111. Thetop photoresist layer 1114 also has openings 1115 and 1118 formedtherein.

FIG. 11( e) illustrates the top and bottom oxide film layers 1111 and1112 after oxide material has been removed in a first etching operation.Material is removed in the openings 1115, 1116, 1117, 1118 in thephotoresist layers 1113 and 1114. Typically, this is accomplished usingetching techniques known in the art. In one embodiment, this etching ofthe oxide layers 1111 and 1112 is accomplished using wet etchingtechniques. As is known to one of ordinary skill in the art, dry etchtechniques can be used.

FIG. 11( f) illustrates the formation of a second top photoresist layer11122. The second top photoresist layer 1122 is formed over remainingtop oxide layer 1111 and over portions of the exposed device siliconlayer 1102 in hinge regions 1120, 1121 (region 1115 of FIG. 11( d)).Certain areas 1118 of exposed device silicon layer 1102 are not masked.The second top photoresist layer 1122 is patterned in the hinge regions1120, 1121 to permit the formation of hinges by etching.

FIG. 11( g) shows the effect of a second etching (material removal)operation. This operation is typically accomplished using etching. Inparticular, reactive ion etching (RIE) or other directional etchingtechniques are preferred. This etch step defines the thickness of hingesin regions 1120 and 1121, and also defines the difference betweenreflector thickness and hinge thickness. With reference to FIG. 11( h)the top photoresist layer 1122 is removed.

FIG. 11( i) illustrates a third etching operation. The top oxide layerserves as a hard mask over the device silicon layer 1102. The exposedregions of the device silicon layer 1102 are etched. In particular, inhinge regions 1120, 1121 (of FIG. 11( g)) and the exposed areas 1118.Such etching should be accomplished using RIE or other directional etchtechniques. In this way the patterned hinge areas 1120, 1121 willmaintain their pattern and maintain their differential thickness withrespect to reflector thickness. The internal oxide layer 1103 serves asan etch stop for the third etch operation.

FIG. 11( j) illustrates a fourth etching (or material removal)operation. The bottom surface of the SOI wafer 1101 is etched throughopenings in the bottom oxide layer 1112. The fourth etch removesmaterial to form a pocket in region 1116 and to define separation linesin region 1117. The material can be removed by etching, preferably usingREI or other directional etching techniques known to those havingordinary skill in the art. Again, the internal oxide layer 1103 servesas an etch stop for the fourth etch operation.

FIG. 11( k) illustrates a fifth etching (or material removal) operation.The fifth etch removes the internal oxide layer 1103 by backsideetching. Etching techniques known to those having ordinary skill in theart may be used. FIG. 11( l) depicts the forming of a reflective layer1129 on one or both sides of the movable optical element 1128. Thereflective layer can be formed using a wide variety of materials andtechniques known to one of ordinary skill in the art. One processincludes forming a metal reflective layer 1129 on at least one of thetop and bottom surfaces of the movable optical element 1128. A suitablemetallization material includes, but is not limited to gold. Adhesionlayers, such as chromium, titanium or tantalum may be employed. A widevariety of deposition techniques can be used to form the metalreflective layers 1129, for example, double sided sputtering.

FIG. 11( m) depicts a sixth etching operation used to remove material inthe region 1117 to complete the separation line 1130. This allows theactuators to be released from the substrates in arrays of desired size.An earlier etching of these lines would lead to a premature separationof the wafer into arrays. Once separated, the separated arrays can thenbe coupled and aligned with a mated wafer having formed thereoninterconnect circuitry, driving electronics, and control circuitry.These completed arrays are hermetically sealed in packages.

The order of the steps can be altered without departing from theprinciples of the invention. The use of oxide masks can be substitutedwith additional photoresist masks. Also, low-stress dielectric materialsin layer 1103 facilitate release of structures from the wafer. Also, itis preferable to use low stress materials for the internal etch stoplayers. Such materials include low stress silicon oxides on the order ofabout 10–100 MPa. Sputtering or plasma enhanced chemical vapordeposition processes that provide very low stress are used rather thanthermally deposited oxides. Because hinges are fabricated from singlecrystal silicon, creep and fatigue are minimized and reliability isimproved as compared with devices that use hinges made with polysilicon,metal and metal alloys in surface micromachining. Rotational combdesigns have leads incorporated on movable electrodes and no bottomelectrodes are required. The interconnections between the top and bottomwafers are fabricated using, for example, solder reflow or othertechniques.

It should be noted that the optical devices formed on the wafers arevery delicate. Care must be taken in separating the wafer into itscomponent arrays. One approach for separating the very sensitiveactuators into individual arrays (dies) is performed in combination ofthree steps. First, separation lines are defined lithographically orwith shadow masking and dry etched, usually using standard deep reactiveion etching of silicon. The etch depth is chosen such that the wafercontaining the actuators retains its rigidity but does not separate intoindividual dies. In the next step, deeper cuts are made along separationlines with laser cutting. The cut depth is controlled by pulse energy,pulse rate, number of pulses and translational speed of the substrate orlaser beam. It is desirable to use lasers with very short pulse durationas shorter pulses reduce size and amount of particulate contamination.In addition, short wavelength lasers are used in order to providesufficient absorption of laser energy by the material desired to be cut.Examples of the appropriate lasers are tripled or quadrupled neodymiumYAG and Ti sapphire. With very short laser pulses, only gaseousby-products form during cutting and thus particulate contamination canbe eliminated. Photochemical laser cutting can also be employed. A smallthickness of material is left remaining in the trenches so thatparticulate and/or gaseous contamination does not collect on the morecritical surfaces (e.g. optical reflecting surfaces) during laser of thedevice. The final step involves cleaving this remaining material with asmall amount of torque applied to separate the arrays. An alternativeseparation process can use only dry etching in combination with cleavingor laser cutting followed by cleaving. A preferred approach includes allthree process steps. Additionally, these techniques, either individuallyor in combination, can be used to effect device separation from both thefront and the backside of the wafer.

The present invention has been particularly shown and described withrespect to certain preferred embodiments and specific features thereof.However, it should be readily apparent to those of ordinary skill in theart that various changes and modifications in form and detail may bemade without departing from the spirit and scope of the invention as setforth in the appended claims. In particular, it is contemplated by theinventors that the various hinge types disclosed herein can beinterchanged in the various array embodiments. Also, the reflector arrayembodiments disclosed herein can be practiced with optical switchembodiments having one, two, three, and more reflector arrays. Also, theprinciples of the present invention may be practiced with reflectorshaving other structures and reflector geometries. Furthermore, theexamples provided herein are intended to be illustrative rather thanlimiting. The inventions illustratively disclosed herein can bepracticed without any element which is not specifically disclosedherein.

1. A method for forming an array of MEMS optical elements, the methodcomprising: providing a single crystal silicon on insulator (SOI) waferhaving a layered structure comprising a silicon wafer layer having aninternal oxide layer formed thereon and having a device silicon layerformed on the internal oxide layer, wherein a top surface of the devicesilicon layer has formed thereon a top oxide layer, and wherein thebottom surface of the silicon wafer layer has formed thereon a bottomoxide layer; forming a bottom photoresist layer on the bottom oxide filmlayer having openings defining a bottom pocket; forming a topphotoresist layer on the top oxide film layer having openings defining ahinge region and open regions; removing the top oxide layer in the hingeand open regions defined by the openings in the top photoresist layer toexpose the hinge region of the device silicon layer; forming a secondphotoresist layer on a top surface of the SOI wafer, the secondphotoresist layer patterning the hinge region of the device siliconlayer so that a hinge can be formed; etching the patterned hinge regionto remove portions of the device silicon layer forming recessed portionsdefining the hinge; removing the second photoresist layer, therebyexposing the underlying top oxide layer as a hard mask layer havingopenings in the hinge and open regions; etching the device silicon layerthrough the openings in the hard mask wherein the recessed portions areetched until the internal oxide layer is reached, and wherein theunetched surfaces are partially etched leaving a portion of the unetchedsurfaces in place to define a thickness of the hinge; etching the bottomsurface of the SOI wafer through openings in the bottom oxide layer toremove material from the silicon wafer layer to form a pocket regiondefining a movable optical element supported by the hinge; and etchingthe SOI wafer to remove the internal oxide layer in the pocket region.2. The method of claim 1, further comprising etching material from aseparation line region between adjacent optical elements to enable thearray to be separated into a plurality of smaller arrays.
 3. The methodof claim 2, wherein etching material from a separation line regioncomprises the operations of dry etching to remove material from theseparation line region; laser cutting in the separation line region; andcleaving in the separation line region into separate arrays of desiredsizes.
 4. The method of claim 2, wherein etching material from aseparation line region comprises the operations of laser cutting in theseparation line region and cleaving in the separation line region intoseparate arrays of desired sizes.
 5. The method of claim 4 whereinetching material from a separation line region comprises the operationsof dry etching to remove material from the separation line region andcleaving in the separation line region into separate arrays of desiredsizes.
 6. The method of claim 4 further comprising assembling theseparated arrays with another wafer having formed thereon appropriatedriving elements and control circuitry, and packaging the assembledarrays.
 7. The method of claim 6 wherein packaging the assembled arrayscomprises hermetically packaging the assembled arrays.
 8. A method forforming an array of MEMS optical elements, the method comprising:providing a wafer having top surface and a bottom surface, the topsurface having formed thereon a top insulating layer and the bottomsurface having formed thereon a bottom insulating layer; forming abottom mask layer on the bottom oxide film layer having openingsdefining a bottom pocket; forming a top mask layer on the top insulatinglayer having openings defining hinge regions and open structures; firstetching to remove the top insulating layer in hinge and open regionsdefined by the openings in the top mask layer exposing a hinge region;forming a second mask layer on the top surface, the second mask layerpatterning the hinge region so that a hinge can be formed; secondetching the patterned hinge region and open region to form a hinge inthe wafer; removing the second mask layer, thereby exposing theunderlying top insulating layer as a hard mask layer having openings inthe hinge and open regions; third etching the wafer through the openingsin the hard mask wherein the recessed portions are etched in a timedetch leaving a portion of the unetched surfaces in place as hinges,thereby defining hinge thickness; fourth etching the bottom surface ofthe wafer through openings in the bottom oxide layer to remove materialfrom the silicon wafer to form a pocket region defining a movableoptical element supported by hinges; and forming a reflective layer onat least one surface of the movable optical element.
 9. The method ofclaim 8 further including a fifth etching to remove material from aseparation line region thereby enabling the structure to be separatedinto arrays of a desired size.
 10. The method of claim 9 wherein theoperation of fifth etching comprises the operations of dry etching toremove material from the separation line region; laser cutting in theseparation line region; and cleaving in the separation line region intoseparate arrays of desired sizes.
 11. The method of claim 9 wherein theoperation of fifth etching comprises the operations of laser cutting inthe separation line region; and cleaving in the separation line regioninto separate arrays of desired sizes.
 12. The method of claim 9 whereinthe operation of fifth etching comprises the operations of dry etchingto remove material from the separation line region; and cleaving in theseparation line region into separate arrays of desired sizes.
 13. Themethod of claim 9 further comprising assembling the separated arrayswith another wafer having formed thereon appropriate driving elementsand control circuitry, and packaging the assembled arrays.